A. Field of the Invention
The invention relates to scanners for providing a digitized image of a document and, more particularly, to a method of manufacturing a highly concentric cylindrical drum for a rotary scanner operable in both a transmissive and a reflective scanning mode.
B. Prior Art
Rotary scanners mount an image source on a drum for scanning by a scanning head. Relative rotational and translational motion between the drum and the head is provided to form successive scan lines. In scanners which are capable of operation in both a transmissive and a reflective mode, the drum is in the form of a thin-walled shell of a translucent material. In the reflective scanning mode, light is reflected from an image source such as a document mounted on the drum into the scanning head for subsequent analysis; in the transmissive scanning mode, light is transmitted through both the drum and through a document mounted on the drum and is thence collected by the scanning head lens for subsequent analysis.
In order to maintain good imaging performance, it is essential to maintain the concentricity of the inside and outside drum diameters about the rotational axis of the drum within a very narrow tolerance range. This not only insures that the distance between the outside surface of the drum and the scanning head remains relatively constant as the drum rotates, but also insures that the illuminating light traverses a relatively constant wall thickness in the transmissive scanning mode. This minimizes changes in the illumination level due to thickness-dependent refraction effects.
Much effort and cost has been expended on a variety of manufacturing techniques for obtaining the desired concentricity. In some cases, for example, specially cast drum stock of controlled dimensions is prepared, and the drum is further worked to obtain the requisite concentricity. Typically, the working includes carefully machining and polishing the inner and outer drum surfaces and repeatedly reworking them .until the desired concentricity is obtained . The cost of these operations, as well as the cost of special drum stock, is generally quite high, and this is reflected in the cost of the resultant end product which may commonly run to several thousands of dollars.
A further constraint on the manufacture of rotary scanner drums is the nature of the drum material itself. An acrylic material is found to have desirable optical properties for scanners, but is relatively soft and thus easily scratched. Accordingly, special care must be taken during the drum manufacturing process to avoid accidental cuts or scratches which destroy the entire blank or at least force major reworking. Further, the presence of "inclusions" such as air bubbles and the like may ruin an otherwise acceptable drum. The result of these factors is frequently drums that are either not wholly satisfactory for the intended purpose, or that are quite high in price.